The present invention relates to a controlling apparatus for an induction motor.
In the general controlling apparatus for an induction motor the induction motor is controlled so as to keep a rotational torque of the motor constant when the motor speed is lower than a base rotational speed of the motor and to keep a output of the induction motor constant when the motor speed is higher than the base rotational speed. The rotational torque constant control is attained by holding secondary magnetic flux constant when the rotational speed of the motor is lower than the base rotational speed and the output constant control is performed by decreasing the secondary magnetic flux in inverse proportion to the rotational speed of the motor when the rotational speed is higher than the base speed. In the controlling apparatus for the induction motor, a terminal voltage of the induction motor is changed depending on a load of the induction motor and a maximum value of the terminal voltage of the induction motor is controlled so as not to exceed a maximum voltage which is output by a cycloconverter connected to the induction motor when the induction motor is driven with a maximum load at maximum rotational speed. Therefore the terminal voltage of the induction motor is controlled having a sufficient margin of the voltage so as to be very low usually and the efficiency of the induction motor becomes very low too. The Japanese published Patent No. 60-14593 (1985) is cited as an example of such apparatus.